In a known manner, typical vehicle steering systems are constructed so that when a driver does not apply steering torque to the steering wheel, and the vehicle is moving, the vehicle steering tends to return to the center, or straight line of travel, position. This return-to-center function is in response to road forces acting on the turned front tires of the moving vehicle that tend to force the turned tires to the neutral (center) position. However, excess friction in the steering system can interfere with the return-to-center function of the steering. One known method to prevent this is to reduce the overall steering ratio of the steering system. This has the effect of increasing steering sensitivity, which is not always desirable.
In electric power steering systems, a known technique for preventing steering system friction from interfering with the return-to-center function is to generate a command, in relation to vehicle speed, that commands the electric power steering actuator to provide a return-to-center position force. The force commanded to the actuator overcomes the steering friction and allows the return-to-center function.
In a known manner, electric power steering motors are controlled, in some systems, in a current mode and, in other systems, in a voltage mode. Current mode control provides certain advantages because the inherent control damping and speed dependability of voltage mode control are eliminated. However, the elimination of the control damping that is present in the voltage mode can prevent the system from obtaining stable closed-loop poles.